This invention relates generally to the stabilization of radio frequency (RF) power amplifiers. Specificially, the present invention prevents the generation of subharmonic spurious outputs in RF power amplifiers.
An output stage of an RF power amplifier may become unstable when it drives a severely mismatched load. A severely mismatched load condition may result from poor antenna or cable connections, broken or otherwise faulty antennas, or proximity of the antenna to electrically conductive surfaces. When this severe mismatch occurs, amplifiers often exhibit an instability in which spurious frequencies are produced. The spurious frequencies may be submultiples, such as one-half, of the fundamental RF output frequency.
RF amplifiers often utilize a transistor in a common emitter configuration for the output stage. The spurious frequencies occur as a result of the output transistor's collector-base diode becoming momentarily forward biased. The output transistor's collector-base diode is reverse biased under normal operating conditions, but may become forward biased when a severe mismatch condition causes the impedance reflected to the output transistor's collector to increase. Resultingly, collector voltage increases, and negative excursions of the collector voltage are so low that the collector-base diode of the output transistor becomes temporarily forward biased.
The forward biasing of the collector-base diode momentarily causes a storage of electrical charge carriers which alters the output transistor's gain parameters. Consequently, the output transistor amplifies a subsequent cycle of an RF signal to a lesser degree than it amplified a previous cycle. The lesser amplified cycle does not cause negative excursions of the collector voltage to forward bias the collector-base diode. Thus, stored charges may clear by the end of the lesser amplified cycle. After the stored charges clear, the greater amplified cycle repeats, causing the output transistor's collector-base diode to become forward biased again. This process continuously repeats causing an undesirable spurious output at a subharmonic of the fundamental RF.
Severe cases in which this subharmonic instability occurs may damage the output transistor. Prior art circuits which protect the output transistor tend to reduce the subharmonic instability in severe cases, but are not effective in reducing spurious outputs generally. Some prior art circuits reduce the amount of power delivered to a mismatched load. The usefulness of the transmitter is severely impaired since it does not deliver the maximum stable RF power it is capable of delivering.
Other prior art circuits sense the amount of power being reflected from an antenna back to an output transistor. Such circuits do not prevent the occurrence of spurious emissions when severe mismatch conditions cause very little power to be delivered to the antenna in the first place.
Other prior art circuits reduce the level of a signal driving the output transistor. Such circuits typically utilize a complex control loop which limits control of the drive signal to the response time of the loop. Due to slow response times, some types of spurious outputs, such as may occur from a loose or intermittent connection, cannot be prevented.